There are previously known apparatus and methods for identifying individuals by scanning their retinal vasculature patterns. U.S. Pat. No. 4,109,237, issued Aug. 22, 1978, and U.S. Pat. No. 4,393,366, issued Jul. 12, 1983 describe using a rotating optical scanning beam to obtain an identification pattern from the fundus of the eye. The earlier scanner employed green light for the scanning beam, whereas the later, improved, scanner employed infrared ("IR") radiation to detect the vasculature of the choroid of the eye. The choroidal vasculature forms a matting behind the retina, including the area of the macula and fovea where retinal blood vessels are very small or nonexistent. The blood vessels of the choroid are stable, as are those of the retina, and thus may be used for obtaining data relating to the identity of an individual. However, because the scanners obtain data from a annular region centered around the optic nerve, the resulting identification pattern is very sensitive to head tilt.
U.S. Pat. No. 4,620,318, issued Oct. 28, 1986 describes an improved apparatus and method for identifying individuals through the ocular light reflection pattern from the fundus of the eye. A fixation beam is provided along the visual axis of the eye, and a scanner obtains an identification pattern from a substantially annular scanning pattern centered around the fovea. As described above, scanning with IR light provides reflections from the vasculature of the choroid as well as the vasculature of the retina. The annular scan centered about the visual axis, and therefore on the fovea, provides substantially identical identification patterns from a given individual regardless of a head tilt angle relative to the visual axis.
FIGS. 1 and 2 show an eye 5 into which the fixation and scanner beams are projected. Eye 5 is centered on visual axis 10 that intersects a fundus 12 at a fovea 14. An optic nerve 16 is located at an angle 17 approximately 15.5 degrees off visual axis 10. Fundus 12 includes a retina 18 and a choroid 20.
FIG. 2 shows an exemplary retinal vasculature 22 branching from the area of optic nerve 16. Retinal vasculature 22 is readily apparent upon observation with either visible or IR radiation. However, when illuminated with IR radiation, the vasculature of choroid 20 also becomes observable as is depicted by the matting of choroidal vessels 24. Choroidal vessels 24 are apparent in the area of fovea 14.
With reference to FIGS. 1 and 2, a collimated IR scanning beam 26 reflects from a combination of retinal vasculature 22, choroidal vessels 24, and various other structures and pigmentation. IR scanning beam 26 enters a pupil 28 of eye 5 and is focused on fundus 12 by a lens 30. IR scanning beam 26 traverses a circular locus of points 32 that are substantially centered around fovea 14.
The prior art scanning apparatus designed to achieve the above-described result is shown in FIG. 3. A fixation target 33 allows an individual to properly focus eye 5 and align its visual axis 10 with an optical axis 34 of the scanning apparatus. Fixation target 33 includes a visible light-emitting diode 35 positioned in a mounting structure 36 having a pinhole 37. Light-emitting diode 35 illuminates a fixation reticle 38 formed by a plate having multiple concentric circles upon which eye 5 is focused.
An IR source 39 provides a beam of IR radiation for scanning fundus 12 of eye 5. IR source 39 includes an incandescent tungsten bulb 40 that produces light that passes through a spatial filter 42 and is refracted by a lens 44. An IR filter 46 passes only the IR wavelength portion of the beam, which then passes through a pinhole 48. The beam is then reflected by a mirror 50 onto a beam splitter 52 that is mounted to coincide with the fixation target optics and optical axis 34.
A portion of radiation emanating from IR source 39 is transmitted through beam splitter 52 and is absorbed in a light trap 54. The remaining radiation is reflected along optical axis 34 to an objective lens 56, which collimates and directs the beam along optical axis 34.
A scanner is provided for directing the beam into the fixated eye from a plurality of sequential, angularly divergent positions. The scanner includes a rotatable housing 57 and scanner optics that rotate with the housing as indicated by a circular arrow 58.
The scanner optics include a hot mirror 59 (one that reflects IR radiation while passing visible light), located in the path of the source beam and the fixation beam. The visible wavelength fixation beam is passed through hot mirror 59, while the IR source beam is reflected away from optical axis 34. A scanner mirror 60 is positioned in housing 57 at a point spaced apart from optical axis 34 and is oriented to direct the IR beam through an IR filter 62 and into eye 5 as housing 57 rotates. Hot mirror 59 causes a displacement of the fixation beam, so an offset plate 64 is positioned to compensate for the displacement.
An objective lens 66 is mounted in an eyepiece 68 to collimate and direct the beam into eye 5. Placing objective lens 66 at this location provides simplified focusing of the device for individuals with other than 20/20 vision.
When housing 57 rotates, the IR beam is directed into eye 5 in an annular scanning pattern centered on the fovea as represented by circular locus of points 32 (FIG. 2). Light reflected from fundus 12 of eye 5 varies in intensity depending on the structures encountered by the scan. The reflected light is recollimated by lens 30 of eye 5, directed out pupil 28, back through objective lens 66 and IR filter 62, and reflected off scanner mirror 60 and hot mirror 59. The reflected beam is then focused by objective lens 56 onto beam splitter 52 which passes a portion of the reflected scanning beam to a hot mirror 70 that reflects the beam through a spatial filter 72. The beam is next reflected by a mirror 74, refracted by a lens 76, and passed through another spatial filter 78 to a detector 80.
Fixation target 33, IR source 39, detector 80, and associated optical components are mounted on a carriage 82. The above-described beams enter and leave carriage 82 coincidentally and focus at optically equal distances from eye 5. Longitudinal movement of carriage 82 serves to focus the device for individuals with other than 20/20 vision. Therefore, when an individual moves carriage 82 longitudinally to focus on fixation target 33, the optics associated with IR source 39 and detector 80 are simultaneously focused. Fixation mounting structure 36 and pinhole 37 are positionable on carriage 82 to provide fine alignment of fixation target 33 and fixation reticle 38 with optical axis 34.
IR source 39 has a fixed intensity, but fixation target 33 has an adjustable intensity by which an individual can optimize the viewability of fixation target 33 through fixation reticle 38. However, changing the intensity of fixation target 33 causes the diameter of pupil 28 to change which can cause variations in the signal received by detector 80. Moreover, individuals are sometimes confused by the image created by fixation target 33 and fixation reticle 38, causing them to improperly align visual axis 10 with optical axis 34.
Other problems with the prior art device of FIG. 3 include difficulty maintaining alignment of the multiple optical components, lack of identification repeatability caused by manual focusing and the confusing fixation target, and expense associated with the optical complexity.
What is needed, therefore, is a substantially simplified optical scanner system having inherent optical alignment, no need for manual focusing, and an improved fixation target.